1. Field of the Invention
The present invention relates to an ion implantation apparatus and an ion implantation method, both of which are necessary to implant an ion beam from an ion source into a wafer.
2. Description of the Related Art
Generally, an ion implantation apparatus of this kind has an ion source, an acceleration and extraction electrode section, a mass analysis unit FIG. 3, an acceleration/deceleration unit, a wafer processing chamber, and the like, all of which are disposed along a beam transportation path. Such an ion implantation apparatus is used for implanting ions into a wafer, such as semiconductor wafer (hereafter, referred to merely as wafer).
Recently, a semiconductor device trends to increase its density and raise its degree of integration. In order to realize such a high integrated semiconductor device, a technique is required for forming a shallow junction within the wafer. Formation of such a shallow junction brings about necessity of an ion implantation apparatus which is capable of implanting an ion into the wafer with very low energy (less than 5 KeV, preferably below 1 KeV).
On implanting ions with very low energy ions, a method is conventionally adopted to obtain a sufficient beam electric current. In this method, initially an ion beam is extracted with higher energy than desired. Then, the energy of the beam is decreased by decelerating the beam by the use of a deceleration electrode (will be simply referred to as decel) adjacent to the wafer after an ion mass is analyzed.
According to inventors"" expenmental studies, it has been found out that ions with undesirable energy are implanted into the wafer and give rise to serious energy contamination, when the ions are implanted into the wafer with very low energy with the conventional deceleration method. Here the xe2x80x9cenergy contaminantxe2x80x9d is defined as a particle that is the same kind of element as a desired ion but has different energy from the desired ion. Such a contaminant particle reaches to a deeper position of the wafer than desired. If the concentration of the contaminant particles exceeds a criterion (typically 1E18/cm3; depends on the structure of a semiconductor device), the juncton depths are determined not by desired ions but by the energetical contaminant particles. Therefore, it is important to decrease the amount of the energy contamination. Because the concentration of the desired ions is fixed at an appropriate value to achieve good device performance, the only way to reduce the amount of the energy contamination is to reduce the ratio of the energy contamination. Here the energy contamination ratio is defined as a ratio of the number of particles implanted with an incorrect energy to the number of ions implanted with the correct energy.
For such reasons, practical use might have not been realized yet about an ion implantation apparatus which is suitable for forming the shallow junction.
It is an object of the invention to realize an ion implantation apparatus which is suitable for forming a shallow junction.
It is another object of the invention to provide an ion implantation apparatus of the type described, which is capable of reducing an occurrence of an energy contamination.
It is still another object of the invention to provide an ion implantation method which is capable of implanting very low energy ions into a wafer, with a contamination reduced.
An ion implantation apparatus to which the present invention is applicable comprises an ion source, an extraction electrode section, a mass analysis unit, a mass analysis slit and a wafer processing chamber. By means of an embodiment of the invention, the ion implantation apparatus is constituted such that an ion implantation is controlled on the basis of a relationship between a beam transport efficiency of an ion beam and an energy contamination ratio in a wafer, and, as a result, the energy contamination ratio in the wafer can be reduced.
Further, an ion implantation apparatus with another embodiment of the invention comprises a deceleration device within a beam line to decelerate the ion beam. With this structure, the decelerated ion beam is controlled on the basis of the relationship between the beam transport efficiency and the energy contamination ratio. in this case, the deceleration capability is optimized on the basis of a measurement of the beam transport efficiency between the deceleration electrode section and the wafer before an implant. Specifically, the energy contamination ratio can be kept less than an allowable value on the basis of an inverse proportional relation between the beam transport efficiency from the deceleration electrode section to the wafer and the energy contamination ratio. The above energy transport efficiency can be measured by two Faraday cups. One is located just after the deceleration electrode section and the other is located just after the wafer. Such measurement of the energy transport efficiency is carried out prior to the beginning of an ion implantation process.
The ion implantation apparatus according to yet another embodiment of the invention is constituted such that a measured value of the beam transport efficiency is compared with a predetermined allowable lower limit. As a result of the comparison, the ion implantation is stopped if the measured value is less than the lower limit. in addition, an equipment for tuning or adjusting both the ion source and a beam transport system is installed.
Further, when the measured value of the beam transport efficiency is insufficient and the implantation process is inhibited from starting, an error message is displayed on the screen of a control computer. Then both of the ion source and the beam transport system are automatically tuned again.
The above-mentioned tuning unit for beam transport includes the mass analysis slit which is selectable in width and which serves to adjust mass resolution. The width selectable slit can work as the deceleration electrode when the decel voltage is supplied to this slit.
The tuning unit for tuning the beam transport system comprises mass analysis magnet as well. The mass analysis slit is automatically set to the minimum width and the beam axis is adjusted by changing a coil electric current of the mass analysis magnet. If this beam axis is shifted from the designed position, the beam transport efficiency is decreased.
As readily understood from the above, the invention measures the beam transport efficiency prior to the beginning of implanting the ion into the wafer, and a predetermined allowable lower limit which corresponds to an allowable energy contamination ratio can be set in each recipe for each implantation.